This invention relates to a method for monitoring particles in a plasma processing chamber.
Since semiconductor devices were first introduced several decades ago, the features sizes of the devices created with these thin films have decreased dramatically in size. During that time, integrated circuits have generally followed the two year/half-size rule (often called "Moore's Law"), which states that the number of devices that will fit on a chip will double every two years. Today's semiconductor fabrication plants routinely produce devices with feature sizes of 0.5 .mu.m or even 0.35 .mu.m, and tommorrow's plants will be producing devices with even smaller feature sizes.
Even as feature sizes have diminished, die sizes have increased. Smaller feature sizes and larger die sizes used in today's semiconductor devices have effectively created a larger target that is susceptible to damage by smaller particles. Because both of these effects reduce yield, particle contamination has become of increasing concern. The presence of particles during deposition of etching of thin films can cause voids, dislocations, or shorts, which adversely affect performance and reliability of the devices fabricated.
The problem of contaminant particles was addressed initially by improving the quality of clean room ambients and employing automated equipment to handle materials and semiconductor substrates. Cleaning of substrate surfaces was also improved. These improvements have reduced the number of particles that exist in the processing ambient or on the substrate surface during the processing cycle. However, particles may be generated inside the process chamber by sources such as the processing materials employed, mechanical contact (e.g., between surfaces in robotic equipment during transfer operations), electrical arcs, and the like.
In particular, during plasma-based substrate processing (e.g., physical-vapor deposition (PVD), plasma-enhanced chemical vapor depositions (PECVD), high-density plasma CVC (HDP-CVD), and similar processes), numerous fragments of various kinds and generated from the process gases employed, including ions, electrons, particles, and the like. The fragments can combine to form particles having small negative charges, i.e., on the order of 10.sup.4 negative elementary charges (an elementary charge is the charge possessed by a single electron). Additionally, residues may accumulate on the processing chamber's interior surfaces during plasma-based substrate processing operations. These residues may include substances such as polymers and compounds formed by the reaction of process gases and the chamber. Stress, such as that due to thermal cycling and the like, may subsequently cause these films to fracture and dislodge from the surfaces on which they have formed, thereby generating particles. Particles may also be generated within the processing chamber during substrate transfer operations, by friction between components, by differences in thermal expansion coefficients, and other sources.
The prior art teaches various techniques for reducing the deposition of particles onto substrates during processing and techniques for removing particles between process steps. However, despite the reduced particle counts provided by these techniques, there remain the need to monitor particle concentrations because particles cannot be completely eliminated from processing chambers. Thus, depending on the process, a processing chamber must be opened and mechanically cleaned (as by wiping) at some point. Because such cleaning operations are disruptive to throughput, they should be performed as infrequently as possible. On the other hand, allowing a chamber to become overly susceptible to contaminating particles can adversely affect yield. Accurate monitoring of particle counts allows optimization of the frequency with which chambers are dismantled for cleaning and also permits the detection of catastrophic failures that might not otherwise be detected as rapidly.
Several types of particle monitoring systems are in use in the semiconductor industry. For example, one type of system monitors particles in the exhaust gas stream by shining a high-intensity laser beam across the gas stream, and detecting scattered light. The amount of scattered light provides a measure of the particle concentration in the exhaust gas, and by implication, a measure of the particle counts within the reaction chamber.
While exhaust gas monitors are presumably effective, the information they provide is second-hand. Preferably, those who operate substrate processing equipment would like to directly measure the particulate concentration within a processing chamber using an in situ technique. This may be done in a manner similar to the aforementioned particle monitoring technique, in which the laser light is sent through the chamber in the space above the substrate being processed. A technique of this sort is described in U.S. Pat. No. 5,328,555, entitled "Reducing Particulate Contamination During Semiconductor Device Processing," issued to A. Gupta. The U.S. Pat. No. 5,328,555 patent is assigned to Applied Materials, Inc., the assignee of the present invention, and is hereby incorporated by reference in its entirety.
The U.S. Pat. No. 5,328,555 patent describes a particle concentration measurement system employing an in situ technique. A laser light-scattering system described therein permits operations such as viewing and measuring particle concentrations within the system's processing chamber. The measurement system's laser is connected to a scanner by a fiber optic cable. The fiber optic cable can be mounted in a laser holder attached to a vertical shaft supported on a rotational base. Any laser whose radiation may be scattered by the particles existing in the processing chamber may be sued.
The scanner is mounted on a holder attached to an adjustable verticle stage to permit the illuminated volume to be positioned at varying heights above the substrate being processed. The laser and scanner slide on an optical rail situated parallel to a chamber viewport. This arrangement provides adjustment in the X, Y, and Z directions. The consistency of the laser light field is controlled by a frequency generator input. It is this oscillation that produces the laser light field. The amount of oscillation (i.e., the size of the laser light field) can be varied by varying the amplitude of the voltage input to the scanner, with a DC offset controlling the starting position of the laser light field's extent.
The laser light field illuminates particles within the processing chamber. Laser light in this field is scattered by these particles, and is then detected by a detection device such as a camera. The camera is configured to provide various focal lengths by adjustment of the viewing distance from a second chamber viewport. Laser light scattered from the particles lying within the camera's field of view may then be accounted for in a variety of ways, such as by display on a monitor, which permits viewing of the particles or measurement of the scattered light's intensity.
Undoubtedly, feature size will continue to decrease, driving the need for further reductions in particle concentrations to maintain yield. Additionally, smaller feature sizes will mean increased sensitivity to defects caused by particles, reducing the particle size necessary to cause a defect and increasing the amount of damage done by a given particle. Thus, even if an in situ technique is employed, accurate measurement of these particle concentrations may prove difficult, due to the particle size and reduced concentrations which must be accurately detected. What is therefore needed is a method of increasing the sensitivity of both existing and future in situ particle concentration sensing techniques.